1. Field of the Invention
The invention relates to hydraulic electronic unit injector (HEUI) assemblies and, more particularly, to a method of convening a non-accumulator-type HEUI assembly to an accumulator-type HEUI assembly and to the injector assembly thus formed.
2. Discussion of the Related Art
Hydraulic electronic unit injector (HEUI) assemblies have gained increased acceptance in recent years because they permit more precise control of fuel injection timing and quantity than is possible with traditional jerk-type injector assemblies and thus can significantly reduce exhaust emissions and improve fuel economy. Both accumulator-type and non-accumulator-type HEUI assemblies are known and both employ pulse width metering and/or pressure metering to control the timing and quantity of fuel injection. As will now be detailed, however, the manner in which energy for injection is stored and released differs fundamentally between the two types of assemblies.
A non-accumulator-type HEUI assembly is commercially available from Caterpillar, Inc. of Peoria, Ill. and is characterized by a needle assembly, a pressure intensifier assembly, and a solenoid actuated popper valve. The solenoid valve is operable to selectively connect a low pressure chamber of the pressure intensifier assembly to a source of fluid pressure and to vent, thus pressurizing or depressurizing a high pressure chamber of the intensifier assembly. The high pressure chamber is fluidically coupled to a fuel supply rail and to a nozzle cavity of the needle assembly. The needle lifts to permit injection whenever fluid pressure in the high pressure chamber increases above a designated level (determined primarily by a needle return spring) and closes whenever the fluid pressure in the high pressure chamber decreases beneath this same level. This type of injector assembly exhibits marked drawbacks and disadvantages.
For instance, injection energy must be transferred very rapidly, i.e., simultaneously with injection. This rapid energy transfer requires an extremely fast acting valve and leads to relatively high parasitic losses. Indeed, for peak injection pressures of about 1200 bar, it is estimated that the injector assembly uses about 5% of engine power.
Secondly, a non-accumulator-type HEUI assembly cannot be used in an expanding cloud injection system (ECI system). An ECI system is one which injects nearly the entire mass of each fuel charge at a decreasing rate such that successive fuel droplets have high separating velocities. Injection in this manner prevents droplet agglomeration and inhibits burning of liquid fuel, thus reducing smoke and emissions. The construction and operation of such an ECI system and its advantages are discussed in detail in a commonly assigned patent application Ser. No. 08/227,868, filed Apr. 18, 1994 in the name of N. John Beck and entitled "Expanding Cloud Fuel Injection System, now U.S. Pat. No. 5,392,745."
In a non-accumulator-type injector assembly, on the other hand, fuel is necessarily ejected at a rate that increases through much of the injection event. Injection at an increasing rate causes successive fuel droplets to travel at higher velocities and leads to droplet agglomeration and undesired liquid fuel burning.
Thirdly, fluid pressure in the nozzle cavity decreases rapidly upon intensifier plunger reversal and accompanying pressure decay, resulting in very rapid and undamped needle closure which can lead to premature wear and failure of the needle and associated valve seat.
Many of the drawbacks of non-accumulator-type HEUI assemblies can be avoided or at least alleviated through the use of accumulator-type HEUI assemblies. An accumulator-type HEUI assembly differs from a non-accumulator-type HEUI assembly primarily in that it permits the energy for fuel injection to be applied prior to the injection event and to be stored at a location near the needle until injection actually takes place rather than being applied during the injection event as in a non-accumulator-type injector assembly. Such assemblies are typically characterized by the use of (1) an accumulator in one-way fluid communication with the intensifier high-pressure chamber and in two-way fluid communication with the nozzle cavity and (2) a control cavity which places the high pressure chamber of the intensifier in two-way fluid communication with the upper surface of the needle plunger. Intensification of fuel pressure in the high pressure chamber forces fuel into the accumulator but does not immediately lead to injection because lifting forces imposed on the needle by accumulator pressure are opposed by an equal pressure in the control cavity. Injection is initiated by de-energizing the solenoid valve to vent the intensifier low pressure chamber and to reverse plunger movement. The resulting pressure decay in the high pressure chamber and control cavity removes opposing forces on the needle and permits accumulator pressure in the nozzle cavity to lift the needle. Fuel injection terminates when lifting forces imposed by fluid pressure in the nozzle cavity drop below closing forces imposed by a needle return spring and by the then-diminished fluid pressure in the control cavity. Accumulator-type HEUI assemblies of this type are disclosed, for example, in U.S. Pat. No. Reissue 33,270 to Beck et al.
Accumulator-type HEUI assemblies can employ much slower acting valves than are required by non-accumulator-type HEUI assemblies because the injection energy can be applied at a relatively leisurely pace prior to injection. Indeed, intensification in an accumulator-type assembly can take place about one-tenth as fast as is required in a non-accumulator-type assembly with about half the parasitic losses. In addition, because injection takes place solely under the control of pressurized fluid trapped in an accumulator which is at a peak value when injection commences, nearly the entire mass of each fuel charge is injected at a steadily decreasing rate. An accumulator-type HEUI assembly thus is readily suitable for use in an expanding cloud injection process. Moreover, because fuel pressure in the nozzle cavity ceases to decay upon needle closure, the needle lifting forces imposed thereby never drop more than slightly below the needle closing force and thus serve to damp needle closure, thereby increasing needle life and reducing the chances of premature failure.
One problem with accumulator-type HEUI assemblies is that they are not at present widely available. Manufacturers of non-accumulator-type HEUI assemblies are reluctant to convert their operations to the production of accumulator-type HEUI assemblies, possibly because wholesale retooling and other expenses heretofore considered to be required for such conversion were considered cost prohibitive. These same concerns heretofore have stifled the conversion of preassembled non-accumulator HEUI assemblies to accumulator HEUI assemblies.